The present invention relates to an electrical switch assembly which is designed for low voltage, high continuous operating current, DC voltage operation. The switch assembly is adapted for use as a parallel path electrical shunt for use across the terminals of electrochemical cells, particularly for diaphragm type cells with operating currents of about 150,000 amperes or greater.
Such an electrochemical cell is discussed in U.S. Pat. No. 4,227,987, and a plurality of cells are typically provided in series with a constant current power supply. The shunt switch assembly is connectable across the terminals of an electrochemical cell to permit the cell to be isolated from the operating system for servicing or replacement without having to shut down the entire system. The shunt switch assembly should be an efficient current bypass device which can be operated to interrupt the very high current and to divert the system current back through the repaired cell.
It had been the practice in the industry to use electrical switches for such shunts or bypass switches which were knife edge contactors or similar air gap contacts. A recent innovation has been to use vacuum shorting switches in a bypass shunting switch assembly as described in U.S. Pat. No. 4,216,359. A multi vacuum switch shunting assembly designed for approximately simultaneous operation of the parallel connecting vacuum switches in described in U.S. Pat. No. 4,302,642 filed Aug. 24, 1977, entitled "Vacuum Switch Assembly," owned by the assignee of the present invention. In the aforementioned copending application generally tubular bus conductors of a predetermined resistance value extend from each vacuum switch to the cell terminals. These tubular bus conductors are closely spaced and aligned to minimize inductance. Another vacuum switch shunting assembly is described in copending application Ser. No. 154,153, filed May 28, 1980, now U.S. Pat. No. 4,370,530, entitled "Electrolytic Cell Electrical Shunting Switch Assembly," owned by the assignee of the present invention. The plurality of parallel connected vacuum switches in the aforementioned copending application each have a series connected resistor and are individually operable with a separate air cylinder.
It is desirable that a shunting switch assembly for use with an electrochemical cell be as compact as possible to minimize bus conductor material costs and inductance effects. The electrical switches of the assembly must be able to efficiently pass the bypass system current without overheating and without undue electrical losses. The electrical switches must be capable of diverting the system current back through the cell and to dissipate the interrupted arc current.
The continued operability and reliability of the switches of the jumper or bypass switch assembly when used with high current electrochemical cell systems is determined by the switch capability to dissipate during contact opening the stored inductive energy of the system. This energy, commonly in the range of 5,000 to 50,000 joules, can produce significant contact wear and erosion. For plural parallel connected bypass switch assemblies, the division of this energy among the plurality of parallel switches requires elaborate current equalizing bus work or great attention to attempts to adjust and synchronize the drive or operating mechanisms for the switches. It is very difficult if not impossible to effectively achieve synchronous switch operation in the needed 0.5 millisecond time scale for such mechanical drive operating systems.